Heat-assisted rotating disk magnetometer for ultra-high anisotropy magnetic measurements

ABSTRACT

An apparatus comprises a spindle to rotate a magnetic recording medium and a magnetic field generator to expose a track of the medium to a DC magnetic field. The magnetic field generator is configured to saturate the track during an erase mode and reverse the DC magnetic field impinging the track during a writing mode. A laser arrangement heats the track during the erase mode and, during the writing mode, heats the track while the track is exposed to the reversed DC magnetic field so as to write a magnetic pattern thereon. A reader reads the magnetic pattern and generates a read signal. A processor is coupled to the reader and configured to measure one or more magnetic properties of the track using the read signal. The apparatus can further comprise a Kerr sensor that generates a Kerr signal using the magnetic pattern.

SUMMARY

Embodiments are directed to an apparatus comprising a spindle configuredto rotate a magnetic recording medium and a magnetic field generatorconfigured to expose a track of the medium to a DC magnetic field. Themagnetic field generator is configured to saturate the track during anerase mode and reverse the DC magnetic field impinging the track duringa writing mode. A laser arrangement is configured to heat the trackduring the erase mode and, during the writing mode, heat the track whilethe track is exposed to the reversed DC magnetic field so as to write amagnetic pattern thereon. A reader is configured to read the magneticpattern and generate a read signal. A processor is coupled to the readerand configured to measure one or more magnetic properties of the trackusing the read signal. The apparatus can further comprise a Kerr sensorconfigured to generate a Kerr signal using the magnetic pattern. The oneor more magnetic properties can comprise magnetic remanence-thicknessproduct (M_(r)t) and Curie Temperature (T_(c)) distribution.

Various embodiments are directed to a method comprising rotating amagnetic recording medium proximate a magnetic field generator, a laserarrangement, and an inductive reader. The method also comprises DCerasing a track of the medium by concurrently exposing the track to afirst DC magnetic field and heating the track with the laserarrangement. The method further comprises writing a magnetic pattern onthe track by concurrently heating the track using the laser arrangementand exposing the track to a second DC magnetic field opposite the firstDC magnetic field. The method also comprises reading the magneticpattern, generating a read signal, and measuring one or more magneticproperties of the track using the read signal. The method can furthercomprise rotating the magnetic recording medium proximate a Kerr sensor,generating a Kerr signal using the magnetic pattern, and measuring oneor more magnetic properties of the track using the Kerr signal. The oneor more magnetic properties can comprise magnetic remanence-thicknessproduct (M_(r)t) and Curie Temperature (T_(c)) distribution.

Other embodiments are directed to an apparatus comprising an arrangementconfigured to rotate a magnetic recording medium configured forheat-assisted magnetic recording. The apparatus also comprises means forDC erasing a track of the medium using a first DC magnetic field, andmeans for writing a magnetic pattern on the track by concurrentlyheating the track and exposing the track to a second DC magnetic fieldopposite the first DC magnetic field. The apparatus further comprises areader configured to read the magnetic pattern and generate a readsignal, and means for measuring one or more magnetic properties of thetrack using the read signal.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The Figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a block diagrams of a heat-assisted rotating disk magnetometer(HARDM) apparatus in accordance with embodiments of the disclosure;

FIG. 2 is a perspective view of a HARDM apparatus in accordance withembodiments of the disclosure;

FIG. 3 is a perspective view of a heat-assisted writer of a HARDMapparatus in accordance with embodiments of the disclosure;

FIG. 4 illustrates various processes of a method for determining one ormore magnetic properties of a magnetic recording disk subject to testingby a HARDM apparatus according to the embodiments shown in FIGS. 1-3;

FIG. 5 illustrates various processes for measuring reversal (switching)probability as a function of laser power using a HARDM apparatus inaccordance with embodiments of the disclosure;

FIG. 6 shows a magnetic pattern written to a single track of a magneticrecording disk subject to testing by a HARDM apparatus in accordancewith various embodiments;

FIG. 7 shows a magnetic pattern that spans several tracks, and is formedby stitching together several neighboring tracks in accordance withvarious embodiments;

FIG. 8 shows data plots representing values of Mrt obtained from readinga stitched track containing a magnetic pattern respectively using aninductive reader and a Kerr sensor according to various embodiments;

FIG. 9 is a graph showing reversal probability as a function of laserpower developed from data acquired by a HARDM apparatus in accordancewith various embodiments;

FIG. 10 shows a modeling result on the effect of Curie Temperaturedistribution developed from data derived from fitting parameters for thecurve shown in FIG. 9;

FIG. 11 shows data plots of reversal probability as a function of laserpower for a ladder of heat-assisted magnetic recording (HAMR) mediadeposited at different temperature in accordance with variousembodiments; and

FIG. 12 illustrates extracted laser power requirement data showinghigher power needed for HAMR media samples grown at higher temperature,and the extracted distributions for such a ladder of HAMR media inaccordance with various embodiments.

The Figures are not necessarily to scale. Like numbers used in theFigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given Figure is notintended to limit the component in another Figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part of the description hereof and in which areshown by way of illustration several specific embodiments. It is to beunderstood that other embodiments are contemplated and may be madewithout departing from the scope of the present disclosure. Thefollowing detailed description, therefore, is not to be taken in alimiting sense.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

Magnetic recording media with extremely high (e.g., ultra-high)anisotropy requires magnetic fields with very high magnetic flux densityto characterize various magnetic properties of the media material. Toensure that magnetic recording disks meet certain standards of quality,these magnetic properties need to be tested from time to time,particularly during the manufacturing process. Heat-Assisted MagneticRecording (HAMR), for example, is a potential recording technology toextend areal density by recording high anisotropy media at elevatedtemperature. Hence, the Curie Temperature (Tc) distribution is veryimportant for media recording performance. At the same time, themagnetic remanence-thickness product (Mrt) is very important for readoutamplitudes and DC noise. However, it remains a challenge to characterizethese important media magnetic properties with good reliability and highthroughput. A conventional rotating disc magnetometer, for example,cannot write HAMR media which has very high coercivity, while ahigh-field VSM (Vibrating Sample Magnetometer) has very low throughputand also causes destructive damage to the disk (i.e., cutting).

Embodiments are directed to a laser-heating assisted electromagneticwriter and reader apparatus and method, with which magnetic patterns(e.g., low frequency patterns) are written on, and read from, highcoercivity media such as those used in HAMR. Embodiments of thedisclosure provide for measuring Mrt, sigma Tc, and other magneticproperties of sample disks having a wide range of coercivity (e.g.,˜5-100 kOe, such as between ˜10 kOe and 50 kOe) with very highthroughput, high reliability, and in a manner that is nondestructive tothe sample disks. Some embodiments are directed to evaluating reversal(switching) probability as a function of laser power in the presence ofa uniform external field. This measurement can be used to extractimportant magnetic properties of the subject disk, such as Tcdistribution. Other embodiments provide for a quick measure of laserpower requirements without having to go through time consumingspin-stand testing.

Various embodiments are directed to a HARDM that utilizes a speciallydesigned heat-assisted writer to record low density waveforms on highcoercivity recording media. This enables, for example, the readout ofM_(r)t values by sensing the magnetic waveforms written to the media.According to some embodiments, a HARDM apparatus includes a laserheating assisted writer, a rotating disk motion platform, a reader(e.g., an inductive reader), and a Kerr sensor. In some embodiments, theKerr sensor is excluded from the HARDM apparatus or is an optionalcomponent. In other embodiments, the reader is excluded from the HARDMapparatus or is an optional component. The HARDM apparatus can beautomated and implemented to include a graphical user interface.

With reference to FIG. 1, a HARDM apparatus 100 is illustrated inaccordance with an embodiment of the disclosure. FIG. 2 is a perspectiveview of an experimental setup of a HARDM apparatus 100 according tovarious embodiments. FIG. 3 is a perspective close-up view ofheat-assisted writing components of the HARDM apparatus 100. The HARDMapparatus 100 includes a spindle 102 configured to rotate a magneticrecording medium 107 relative to a heat-assisted writer 101, aninductive reader 110, and a Kerr sensor 120 (which may be an optionalcomponent according to some embodiments). The heat-assisted writer 101includes a writer 104 and a magnetic field generator 105, such as anelectromagnet. The magnetic field generator 105 can be implemented togenerate a magnetic field of up to about 1.5 Tesla perpendicular to thesurface of the medium 107. The magnetic field generator 105 may be builtinto the writer 104 or be attached or placed in proximity to the writer104. The magnetic field generator 105 is configured to expose a portion109 (e.g., a track) of the medium 107 to a DC magnetic field, such asduring an erase mode and a writing mode.

The heat-assisted writer 101 includes a laser arrangement 103 configuredto heat the medium portion 109 (e.g., track) rotating through the writer104. The laser arrangement 103 includes a heating laser 106 opticallycoupled to beam shaping optics 108, which serve to focus a light beamexiting an output 111 of the laser arrangement 103 onto the mediumportion 109. Adjustable laser beam shaping optics 108 are incorporatedinto the laser arrangement 103 for achieving a tight focus on the mediasurface. With a high power laser source (e.g., several Watts), media canbe heated to and above its Curie Temperature. High coercivity media,such as those used in HAMR, have a much lower coercivity at elevatedtemperature, allowing easy switching under a moderate external magneticfield (e.g., <1.5 Tesla). By adjusting laser power and/or laser pulsewidth and/or duty cycle, temperature of the media can be controlled.

During an erase mode, the magnetic field generator 105 is configured togenerate a DC magnetic field that saturates the medium portion 109 whilethe laser arrangement 103 heats the medium portion 109. This serves toorient the magnetic grains of the medium portion 109 in a specifieddirection (e.g., down). During a writing mode, the magnetic fieldgenerator 105 is configured to reverse the DC magnetic field impingingthe medium portion 109 while the laser arrangement 103 heats the mediumportion 109. This serves to orient the magnetic grains of heated regionsof the medium portion 109 in the opposite direction (e.g., up), therebywriting a magnetic pattern to the medium portion 109.

The HARDM apparatus 100 further includes a reader 110 configured to readthe magnetic pattern written to the medium portion 109. The reader 110is shown positioned proximate the spindle 102, but can be locatedelsewhere. The reader 110 generates a read signal in response to readingthe magnetic pattern written to the medium portion 109. In someembodiments, the reader 110 is implemented as an inductive reader. Inother embodiments, the reader 110 can be implemented using other readtransducer technologies (e.g., a magnetoresistive reader). An inductivereader 110 can be employed in the HARDM apparatus 100 to provide a moreaccurate readout of the total magnetic moment of the medium portion 109as compared to that provided by the Kerr sensor 120, as is furtherdiscussed hereinbelow. A processor 140 is coupled to the reader 110 andconfigured to measure one or more magnetic properties of the mediumportion using the read signal. The processor 140 can be coupled to adisplay 150 (or graphical user interface) with which variousmeasurements and data can be presented and generated. The read signaland/or data developed from the read signal can be output to anotherdevice or system.

In some embodiments, the HARDM apparatus 100 includes a Kerr sensor 120,in addition to or exclusive of the inductive reader 110. The Kerr sensor120 includes a probe laser 122 (which produces incident beam 123 at apower of a few milliwatts), linear polarizer 124, beam splitter 126,polarizing prism (e.g., Wollaston prism) 127, photodiodes 128A and 128B,and differential amplifier 130. In some implementations, thedifferential amplifier 130 can be coupled to a lock-in amplifier (notshown). The lock-in amplifier, using the output signal 132 from thedifferential amplifier 130, can be configured to determine the real andimaginary components of the signal 132 and generate an output signalrepresentative of same. During testing, incident beam 123 produced bythe laser 122 passes through the polarizer 124 and beam splitter 126.Incident beam 123 is reflected at the surface of the disk 107 andundergoes a polarization alteration due to the magnetization of the disk107. A reflected beam 125 is reflected by the beam splitter 126 to adetection arm which includes the Wollaston prism 127 and photodiodes128A and 128B.

The Wollaston prism 127, or other analyzer, is used to separatepolarized beam components 129A and 129B of the reflected beam 125.Photodiodes 128A and 128B provide positive and negative input signals todifferential amplifier 130. The output of differential amplifier 130 isa signal 132 representing a difference in intensity of components 129Aand 129B of the reflected beam 125, which is proportional to the changeof magnetization at the magnetic pattern region of the disk 107. Ifincluded, the lock-in amplifier, using the output signal 132 fromdifferential amplifier 130, is configured to determine the real andimaginary components of the signal 132 and generate an output signalrepresenting same.

After writing a magnetic pattern (e.g., a low frequency square wavepattern) using the laser-heating assisted writer, a Kerr sensor can beused to read out the magnetic signal. Using the Kerr signal 132, theprocessor 140 can produce various data about the magnetic properties ofthe portion of the medium 109 written with the magnetic pattern. Suchmagnetic properties include once-around variation and remanentmagnetization, for example.

FIG. 4 illustrates various processes of a method for determining one ormore magnetic properties of a magnetic recording disk subject to testingby a HARDM apparatus 100 according to the embodiments shown in FIGS.1-3. Initially, a disk 107 is placed on the spindle 102 such that thedisk 107 passes under the output 111 of the laser arrangement 103. Asthe disk 107 passes within the writer 104, the portion 109 passing underthe output 111 of the laser arrangement 103 can concurrently be subjectto a DC magnetic field produced by the magnetic field generator 105. Acircumferential region of the disk 107 passing under the output 111 ofthe laser arrangement 103 defines a track 113 to be written by theheat-assisted writer 101 and read by one or both of the reader 110 andthe Kerr sensor 120.

According to the method illustrated in FIG. 4, the output 111 of theheat-assisted writer 101 is positioned 402 relative to the track 113,such as the center of the track 113. In some embodiments, theheat-assisted writer 101 can be configured for movement relative to astationary spindle 102. In other embodiments, the heat-assisted writer101 can be stationary, and the spindle 102 can be configured formovement relative to the stationary writer 101. In further embodiments,both the writer 101 and the spindle 102 can be configured for movement.It is noted that movement of one or both of the writer 101 and thespindle 102 can be implemented to be fully automatic (e.g., computer andmotor control), fully manual (e.g., operator control), or partiallyautomatic/partially manual. It is also noted that disk 107 is typicallya blank disk devoid of pre-written servo tracks, and that encoders onthe motion stages of the apparatus 100 can be used to locate tracks onthe disk 107 as they are written.

With the writer output 111 properly positioned relative to the disk 107,the track 113 is subject to DC erasure 404. DC erasure 404 of the track113 involves concurrently subjecting the track 113 to heat (e.g.,between about room temperature and about 1000° C.) and a DC magneticfield (e.g., <1.5 T) sufficient to saturate the track 113, such that themagnetic grains of the track 113 are oriented in a down direction. DCerasure 404 can be performed in other ways, such as by the applicationof a high magnetic field (e.g., >6 T) without the need to heat the disk107. The erasure process requires a full disk revolution to complete,which can take about 0.25 seconds, for example. After completing theerasure process, the HARDM apparatus 100 can be transitioned from theerasure mode of operation to a write mode.

Writing to the track 113 using the HARDM apparatus 100 involvesreversing 406 the DC magnetic field to the direction (e.g., up) oppositethat used during the DC erasure process. While subjecting track 113 tothe opposite DC magnetic field, the laser arrangement 103 is modulatedon and off at a desired frequency to write a magnetic pattern to thetrack 113. The magnetic pattern written to the track 113 can be a lowfrequency square wave, such as one between about 1 and 100 flux changesper inch (fci), for example. The region of the track 113 passing throughthe writer 104 during laser-on periods will be magnetized up, while theregion of the track 113 passing through the writer 104 during laser-offperiods will remain magnetized down. This results in the successfulwriting of a single track 113. The track width can be adjusted byadjusting the optical focus, and the pattern frequency can be adjustedby adjusting the laser modulation frequency. The writing processrequires a full disk revolution to complete, which can take about 0.25seconds, for example. As such, the erasing and writing processes for asingle track 113 can take about 0.5 seconds.

Depending on the magnetic properties being evaluated and the type ofsensing used to read the magnetic pattern written to the track 113, asingle track 113 may be sufficient or multiple tracks 113 may be neededfor the particular evaluation. A check 408 is made to determine if thetrack 113 written to the disk 107 is wide enough for readout by theparticular sensor being used for a particular evaluation. For example,if the Kerr sensor 120 is to be used for a specified magnetic propertyevaluation, a single track 113 (i.e., the width of a single track)containing the magnetic pattern may be sufficient. If an inductivereader 110 is to be used, several tracks may be needed to providesufficient readout for the reader 110. If the existing track or tracks113 are not sufficiently wide, the writer output 111 can be positioned410 to a track adjacent the one or ones that have been previouslywritten The erasing and writing processes 404 and 406 can be repeatedfor the adjacent track or tracks. After completion of the writingprocess, and after a track(s) of sufficient width has been written tothe disk 107, one or both of the reader 110 and Kerr sensor 120 can read412 the magnetic pattern written to the track(s). A processor 140 maythen operate on the signal produced by one or both of the reader 110 andKerr sensor 120 to determine one or more magnetic properties of the disk107.

According to some embodiments, the processes illustrated in FIG. 4 canbe implemented in a method for determining the value of M_(r)t for amagnetic recording disk subject to testing by a HARDM apparatus 100. ForM_(r)t reading using an inductive reader 110, for example, usuallymultiple neighboring tracks 113 need to be written to form a track wideenough to provide a signal useful for the reader 110 (e.g., a signalwide enough to provide a good signal-to-noise ratio). According to someembodiments, a number of tracks 113 can be “stitched” together to form asingle or composite track having a width that can be readily detected bythe reader 110.

FIG. 6 shows a magnetic pattern 602 written to a single track which, inthis illustrative example, has a track width of about 0.4 mm. FIG. 7shows a magnetic pattern 702 that spans several tracks, and is formed bystitching together several neighboring tracks. In the illustrativeexample shown in FIG. 7, several tracks are stitched together for form awide track having a width of about 3.5 mm. By way of example, 10 narrowtracks can be being stitched together to form a relatively wide >3 mmtrack, taking about 5 seconds to create (2.5 seconds of erasure time and2.5 seconds of writing time). After stitching tracks, the disk 107 canbe spun to a speed for inductive readback. For example, the disk 107 canbe rotated at a speed of between about 5 and 500 RPM during theerasure/writing phases, and then increased to between about 1,000 and10,000 RPM during the readback phase.

Example M_(r)t data is shown in FIG. 8. FIG. 8 shows M_(r)t evaluationresults on a group of samples with increasing magnetic layer thickness.The data plot 802 represents values of M_(r)t obtained from reading astitched track containing a magnetic pattern using an inductive reader110. The data plot 812 represents values of M_(r)t obtained from readinga stitched track containing a magnetic pattern using a Kerr sensor 120.It can be readily seen that M_(r)t readback using an inductive reader110 is much more reliable in determining the total magnetic moment thanwhen using a Kerr sensor 120. For example, it can be seen that theM_(r)t values for the data plot 802 are linearly proportional tomagnetic layer thickness, unlike the Kerr signal data plot 812. This isbecause the Kerr sensor 120 relies on optical properties of the media,which can be changed by layer structure, material used, wavelengthdependence, and many other factors.

FIG. 5 illustrates various processes for measuring reversal (switching)probability as a function of laser power using a HARDM apparatus 100 inaccordance with embodiments of the disclosure. According to variousembodiments, with a constant magnetic field applied, laser power isincremented to achieve higher and higher temperature at the media.Remanent magnetization is recorded after each increment in laser power.The recorded remanent magnetization as a function of laser power can beused to extract and analyze Curie Temperature distribution of the media.

According to the method illustrated in FIG. 5, the output 111 of theheat-assisted writer 101 is positioned 502 relative to the track 113,such as the center of the track 113. Initially, the power of the laserarrangement 103 is set to a low level, such as a level below thatrequired to change the orientation of the magnetic grains of the track113. With the writer output 111 properly positioned relative to the disk107, the track 113 is subject to DC erasure 504, in which both heat anda uniform DC magnetic field are applied to the track 113. Whilesubjecting track 113 to a DC magnetic field opposite that used duringthe erasure process, the laser arrangement 103 is modulated on and offat a desired frequency to write 506 a magnetic pattern (e.g., a lowfrequency square wave) to the track 113.

The method of FIG. 5 further involves performing a readout 508 using theinductive reader 110 or the Kerr sensor 120. The readout data can beused to generate data such as that shown in FIG. 9 (i.e., reversalprobability vs. laser power). A check 510 is made to determine if thetrack 113 is saturated. If not, the power of the laser arrangement 103is increased slightly (e.g., by about 1%). The processes of blocks 504,506, 508, and 510 are repeated until it is determined that the track 113is saturated. When no further change is observed in the remanentmagnetization upon increasing laser power, the track 113 is consideredsaturated. The readout data is then analyzed 514. For example, remanentmagnetization is measured and recorded after writing with each incrementin laser power, and this data is readout and analyzed.

FIG. 9 is a graph showing reversal probability as a function of laserpower. The data of FIG. 9 represents analysis results produced at block514 of FIG. 5. The reversal probability versus laser power curve of FIG.9 can be fitted with a nonlinear function such as a bi-error function.FIG. 9 shows such a fitting to the experimental result, with excellentaccuracy. The black squares 902 are experimental data, and the solidline 912 is a nonlinear fit using bi-error functions. The derivedfitting parameters provide important information such as sigma Tc andlaser power requirement. For example, to quantify the sharpness of thereversal probability curve such as that shown in FIG. 9, nonlinearfitting based on error functions can be used to fit the experimentalcurve. From the fitting parameters, one can extract the standarddeviation and peak position. These fitted parameters can then be used toderive important metrics such as sigma Tc and laser power requirement.

FIG. 10 shows a modeling result on the effect of sigma Tc. As is clearlyillustrated, a larger sigma Tc results in a much shallower reversalprobability slope. Therefore, the steeper the curve is, the narrower thedistributions are. For example, assuming the media's temperature risefrom ambient, T_(amb), is linearly proportional to the heating laserpower P:T−T _(amb) =αP

From the above, the following relationship between

$\frac{\sigma\; T\; c}{T\; c}$and

$\frac{\sigma\; P}{P\; 50}$can be obtained:

$\frac{\sigma\; T_{c}}{T_{c}} = \frac{{\alpha \times \sigma}\; P}{T_{amb} + {\alpha \times P_{50}}}$Where P₅₀ is the laser power required to achieve 50% reversalprobability. Assuming P₅₀ corresponds to Tc, the following can beobtained:T _(c) −T _(amb) =αP ₅₀

Combining this equation with the equation above, the following isobtained:

$\frac{\sigma\; T_{c}}{T_{c}} = {\left( {1 - \frac{T_{amb}}{T_{c}}} \right)\frac{\sigma\; P}{P_{50}}}$

Using 300 K for T_(amb) and 700 K for T_(c), the following is obtained:

$\frac{\sigma\; T_{c}}{T_{c}} \approx {0.57 \times \frac{\sigma\; P}{P_{50}}}$

With this last equation, sigma Tc can be easily derived from themeasured sigma laser power and P50 laser power, both of which can beobtained by fitting the reversal probability curve of FIG. 9.

FIGS. 11 and 12 show the analysis results on a group of samples withvarying deposition temperature. FIG. 11 shows data plots of reversalprobability as a function of laser power for a ladder of HAMR mediadeposited at different temperature. FIG. 12 illustrates extracted laserpower requirement data showing higher power needed for HAMR mediasamples grown at higher temperature, and the extracted distributions forsuch a ladder of HAMR media. It can be seen in FIG. 12 thatdistributions narrow down as growth temperature is increased. The dataof FIGS. 11 and 12 demonstrates that higher deposition temperaturefacilitates better growth and film quality, which then results insmaller distributions. The measurement results shown in FIGS. 11 and 12clearly reflect this trend, demonstrating the accuracy and reliabilityof both the disclosed measurement method and apparatus. For a morein-depth analysis, as is reflected in FIG. 12, a bi-error function canbe used instead of a single error function. The two halves of thereversal probability curve can be fitted differently, deriving twostandard deviation values, sigma Laser left, and sigma Laser right.While the left side standard deviation can be affected by distributionsin the media's anisotropy and grain size, the right side standarddeviation is more closely related to the media's Tc distribution.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations can besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present disclosure. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisdisclosure be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. An apparatus, comprising: a spindle configured torotate a magnetic recording medium; a magnetic field generatorconfigured to expose a track of the medium to a DC magnetic field, thegenerator configured to saturate the track during an erase mode andreverse the DC magnetic field impinging the track during a writing mode;a laser arrangement configured to heat the track during the erase modeand, during the writing mode, heat the track while the track is exposedto the reversed DC magnetic field so as to write a magnetic patternthereon; a reader configured to read the magnetic pattern and generate aread signal; and a processor coupled to the reader and configured tomeasure one or more magnetic properties of the track using the readsignal.
 2. The apparatus of claim 1, wherein the reader comprises aninductive reader.
 3. The apparatus of claim 1, wherein the one or moremagnetic properties comprises magnetic remanence-thickness product(M_(r)t).
 4. The apparatus of claim 1, wherein the one or more magneticproperties comprises total magnetic moment.
 5. The apparatus of claim 1,wherein the one or more magnetic properties comprises a reversalprobability as a function of power applied to the laser arrangement. 6.The apparatus of claim 5, wherein the processor is configured todetermine a Curie Temperature (T_(c)) distribution of the track usingthe reversal probability as a function of applied laser arrangementpower.
 7. The apparatus of claim 5, wherein the processor is configuredto determine a laser power required to achieve a specified reversalprobability.
 8. The apparatus of claim 1, wherein the magnetic patternspans a plurality of stitched-together tracks.
 9. The apparatus of claim1, further comprising a Kerr sensor configured to generate a Kerr signalusing the magnetic pattern.
 10. The apparatus of claim 9, wherein theprocessor is configured to measure one or more magnetic properties ofthe track using the Kerr signal.
 11. The apparatus of claim 1, whereinthe magnetic recording medium has a coercivity of between about 5 kOeand 100 kOe.
 12. A method, comprising: rotating a magnetic recordingmedium proximate a magnetic field generator, a laser arrangement, and areader; DC erasing a track of the medium by concurrently exposing thetrack to a first DC magnetic field and heating the track with the laserarrangement; writing a magnetic pattern on the track by concurrentlyheating the track using the laser arrangement and exposing the track toa second DC magnetic field opposite the first DC magnetic field; readingthe magnetic pattern and generating a read signal; and measuring one ormore magnetic properties of the track using the read signal.
 13. Themethod of claim 12, wherein the one or more magnetic propertiescomprises magnetic remanence-thickness product (M_(r)t).
 14. The methodof claim 12, wherein the one or more magnetic properties comprises totalmagnetic moment.
 15. The method of claim 12, wherein the one or moremagnetic properties comprises a reversal probability as a function ofpower applied to the laser arrangement.
 16. The method of claim 15,further comprising: computing a curve of reversal probability as afunction of applied laser arrangement power; deriving fitting parametersfor the curve; and determining a Curie Temperature (T_(c)) distributionof the track using the fitting parameters.
 17. The method of claim 15,further comprising: computing a curve of reversal probability as afunction of applied laser arrangement power; deriving fitting parametersfor the curve; and determining a laser power required to achieve aspecified reversal probability using the fitting parameters.
 18. Themethod of claim 12, wherein writing the magnetic pattern comprises:writing the magnetic pattern to a plurality of tracks; stitchingtogether the plurality of tracks to produce a stitched track; andreading the stitched track to generate the read signal.
 19. The methodof claim 12, further comprising: rotating the magnetic recording mediumproximate a Kerr sensor; generating a Kerr signal using the magneticpattern; and measuring one or more magnetic properties of the trackusing the Kerr signal.
 20. The method of claim 12, wherein the magneticrecording medium has a coercivity of between about 5 kOe and 100 kOe.21. An apparatus, comprising: an arrangement configured to rotate amagnetic recording medium configured for heat-assisted magneticrecording; means for DC erasing a track of the medium using a first DCmagnetic field; means for writing a magnetic pattern on the track byconcurrently heating the track and exposing the track to a second DCmagnetic field opposite the first DC magnetic field; a reader configuredto read the magnetic pattern and generate a read signal; and means formeasuring one or more magnetic properties of the track using the readsignal.
 22. The apparatus of claim 21, wherein the one or more magneticproperties comprises one or more of magnetic remanence-thickness product(M_(r)t), total magnetic moment, and a reversal probability as afunction of power applied to a laser arrangement used to heat the track.